From Longman Dictionary of Contemporary Englishoutdoorout‧door /ˌaʊtˈdɔː◂ $ -ˈdɔːr◂/ ●●○ adjective1[only before noun]DLOOUT/OUTSIDEexisting, happening, or used outside, not inside a building opp indoora huge outdoor marketoutdoor recreational activitiesoutdoor clothinga healthy outdoor life2 →outdoor typeUSAGE: Outdoor, outdoorsOutdoor is an adjective and outdoors is an adverb.You say: an outdoor swimming pool✗Don't say: an outdoors swimming poolYou say: They could play outdoors.✗Don't say: They could play outdoor.